"NOTE 7 WATER KNOCK-OUT CONTROLS for HEATER-TREATERS FREE WATER"
Process Measurement & Control Solutions NOTE 7: WATER KNOCK-OUT CONTROLS for HEATER-TREATERS & FREE WATER SEPARATORS INTRODUCTION Produced oil contains water in highly variable amounts. This process is complicated by the presence of complex Heater-treaters heat the produced ﬂuid to break oil/water emulsions that do not give a clear cut oil/water interface. emulsions and to reduce the oil viscosity. The water is then typically removed by utilizing gravity to allow the free water A typical installation on a separator has an AGAR ID-201 to separate from the oil. The water is removed through Interface Detector inserted horizontally 24 inches above the a dump valve on the bottom of the separator. Oil in the vessel’s water draw-off nozzle through a 1” - 2” ball valve discharged water is a loss of valuable product and causes and a AGAR Seal Housing. The AGAR ID-201 Interface an increase in water treatment costs. Detector controls the on/off dump valve when the water concentration at the ID-201’s probe tip exceeds a preset Control of the water dump valve is complicated by the value, such as 80%. The setting for the probe is checked nature of the produced ﬂuids, such as those from ﬁreﬂood by analysis of samples taken from the seal housing. oil ﬁelds. Fireﬂooding is a method of extracting very Additional checks are made by taking samples above viscous oil by burning about 10% of the oil below ground and below the seal housing. Since oil/water emulsions to heat the remaining oil to reduce its viscosity. Water containing more than 80% water are not stable, the AGAR injection is then used to push the oil to the producing well. ID-201 Interface Detector control ensures that only oil-free The produced ﬂuid contains highly variable amounts of water is dumped from the vessel and that emulsions build water, carbon (from the burnt oil), oil and sand. It is very above the probe. viscous, forms stabilized emulsions and severely fouls/ coats instrumentation. In the installation shown below, an AGAR ID-201/RL/PN system solenoid valve directly controls the pneumatic- AGAR WATER KNOCK-OUT CONTROL actuated, fail-closed water dump valve. The presence of Both free water knock-out drums and heater-treaters are 20% or more oil will vent the actuator to close the dump gravity separators. Water sinks to the bottom, and oil ﬂoats valve. Water content over 80% opens the solenoid valve, to the top because of density differences. The separator supplies air to the dump valve and opens the water valve, must be large enough to allow the water sufﬁcient residence thus dumping oil free water. The opening and closing time and laminar conditions that enable the water to settle speed of the dump valve is controlled by the air supply to the bottom instead of being carried overhead with the valve for opening, and the solenoid’s throttling when closing oil. the valve. APPLICATION NOTE 7 All Agar Corporation Instruments are covered by one or more of the following U.S. Patents: 4,503,383; 4,774,680; 5,099,697; 5,101,163; 5,101,367; 5,263,363; 5,503,004; 5,551,305; 5,589,642; 5,741,977, RE 36,597. Other patents pending in the USA and other countries. APPLICATION NOTE 7 - Water Knock-Out Interface Detector Installation Details Notes On Interface Float Chambers And Level Gauge Glasses 1. Free gas is seen by the AGAR ID-201 Interface Detector as oil. To avoid false readings, the ﬂuid at the probe’s Most vessels are ﬁtted with level gauges or ﬂoat chambers antenna must be gas free. to give a visual indication of the height of the interface in the vessel. These are mounted in bridles that are separate from 2. The ID-201 is not affected by coatings because it the vessel so that they can be isolated for maintenance. operates at high frequency and can “see” through However, the readings from a gauge glass or a float normal coating build-up. Because velocities inside chamber must be interpreted with caution because: the vessel are low, oil ﬁlms tend to form to a maximum thickness and remain stable, so routine recalibration is 1. The connections between the level gauge and the usually not necessary. vessel usually are located so that emulsions can not enter. Instead of accurately being ﬁlled with oil, an 3. The ID-201 is calibrated on-site using the actual oil and emulsion layer, and water, the level gauge will be ﬁlled process water. If ﬂuids other than oil and process water with only oil and water layers. Moreover, the narrow are to be used, the AGAR factory should be informed tube acts as a separator and shows only clear-cut so that any required modiﬁcations can be made before interfaces. shipment to the end user. 2. The level bridle is actually a manometer showing a 4. Normally, changes in oil and water characteristics will pressure balance between two columns of ﬂuids: one not affect the ID-201. If, however, the water’s NaCI inside the vessel and one outside. Having the same content falls below 2000 PPM, the probe will have to interface height in the chamber as in the vessel is only be recalibrated. coincidence. 5. The ID-201 probe can be adjusted to work within 3 3. Level gauge connections are sometimes located so that inches of grounded metal. However, Agar recommends the gauge can only ﬁll with water. Over time, oil and that no metal be closer than 6 inches to the tip of the emulsion will become trapped at the top of the gauge probe. glass. This gives a false indication of water height. The height and density of the trapped oil will affect the 6. Rapid cycling of the dump valve is usually not a water height shown. problem because the ID-201 incorporates a 5% water content hysteresis, and an approximate 3 second 4. A change in ﬂuid densities in the vessel will affect the delay. However, if rapid valve cycling is a problem and interface height shown in the level gauge. Temperature restriction of the drain rate is not acceptable, there are differential from the vessel to the gauge, changes in three alternatives: (a) Vertical mounting of the Interface oil density, emulsion composition, or water salinity will Detector will increase the level change needed to trigger all affect the gauge’s level. the relay; (b) A time delay relay may be ﬁtted to the ID- 201 to reduce cycling; (c) Two ID-201s can be used -- a 5. Particles in the oil can cause a blockage of the small high probe and a low probe. The dump cycle begins openings in the isolation valves for a level gauge. when water covers a high probe and ends when the This blockage, which prevents the level gauge from water drops below the low probe - System 2. functioning, is not detectable unless the level gauge is purged. USA CAYMAN ISLANDS VENEZUELA MALAYSIA UNITED ARAB EMIRATES INDONESIA Agar Corporation Agar Corporation Ltd. Agarcorp de Venezuela C.A. AgarCorp SDN. BHD. AgarCorp Middle East PT AgarCorp Indonesia 5150 Tacoma Drive P.O. Box 10206 Edif. First, Piso 1, Local 1-B 168-1st Fl Main Rd Salak So P.O. Box 41296 Jalan Teratai CB-17 Houston, TX 77041 Grand Cayman, BWI Calle 75 con Av. 13-A 57100 Kuala Lumpur Abu Dhabi, UAE Ciputat Baru, Ciputat Tel: (832) 476-5100 KY1-1002 Maracaibo, Edo. Zulia Tel: 603-7980-7069 Tel: 971-2-6811150 Tangerang 15413 Fax: (832) 476-5299 Tel: (345) 945-5242 Tel/Fax: +58 261 7978646 Fax: 603-7980-5369 Fax: 971-2-0811779 Tel: 62 21 7409206 ACI@agarcorp.com ACL@agarcorp.com ADV@agarcorp.com ACSB@agarcorp.com ACAD@agarcorp.com PTAI@agarcorp.com www.agarcorp.com ER No. 5024; SL0017 Rev -